What they are talking about is size of an opened file in TIFF format, not the Megapixels of the camera. My experience with Ad Agencies is that they have almost no working knowlege of digital photography, and routinely request obcenely huge files for the smallest of jobs.
What I would do would be to ask them what size the image is going to run, and in what kind of medium. If it's going to be a newspaper ad, web page, or short run press, then don't resize the image. They'll have to size it down, and the image is going to completly fall apart. If it's a poster, board advertising, or a high quality magazine run, then you'll probably have to do some work to it.
David Buzzard